1. Field of the Invention
The present invention relates to an isolator for use in a portable mobile communication unit such as a portable phone.
2. Description of the Related Art
Isolators have characteristics in which attenuation is very low in the direction in which a signal is transferred and it is very high in the reverse direction. They are employed in transmitting and receiving circuit sections or the like of equipment such as portable telephones. As shown in FIG. 6, in such an isolator, three central conductors 30 are disposed so that they intersect each other at the angles shown in an electrically insulated condition. At one end, each of the central conductors 30 is connected to a port P1, a port P2, or a port P3. Each port is connected to a matching capacitor C, the other end of which is connected to ground. A ferrite body 31 butts against the intersection of the central conductors 30 and a DC magnetic field is applied to the intersection. A terminating resistor is connected to one of the ports, in this case port P3.
In the isolator, the angle formed by any two of the central conductors 30 is set to 120 degrees (with an actual machining tolerance of .+-.1 degree) in design, and the resistance of the terminating resistor connected to the terminating port P3 is set to about 50 .OMEGA..
There is a strong demand for a portable telephone to have low power consumption in order to achieve a long continuous call time, while also being compact. It has further been strongly demanded that the insertion loss of an isolator be reduced (to a low level).
A conventional isolator has been designed for an analog portable telephone which uses a nonlinear power amplifier and good isolation (high attenuation in the reverse direction) has been required. An isolation of about 10 to 15 dB is needed whereas the isolation loss is about 0.5 dB. Therefore, to obtain the most suitable isolation characteristic, the three central conductors are disposed with an intersection angle of 120 degrees in the conventional isolator. This setting of the intersection angle is, however, a large burden in terms of insertion-loss reduction.
In a digital portable telephone, a linear power amplifier is used. Since intermodulation distortion caused by an external signal is unlikely to occur, it is sufficient for an isolator to have a small insertion loss, in order to achieve a low power consumption, even if its isolation is lower than that of the conventional isolator.